Environmental continuous air monitors (ECAMs) are becoming much more important to National defense programs in light of recent world events. The specter of the use of “dirty” nuclear devices or radiological dispersal devices (RDDs) by terrorists has become a real possibility. To handle the consequences of such attacks, high volume environmental continuous air monitors must be reliable, and provide service over long periods of time without requiring frequent maintenance. However, a problem with current environmental continuous air monitor designs is that sampled ambient air may contain large quantities of fine particles (particles in the size range less than about 2.5 micrometers aerodynamic diameter) and smoke particles (particle diameters of between about 0.1-1.0 micrometers), which can cause rapid filter clogging and subsequent premature shutdown. This is especially true of ambient atmospheres in the aftermath of RDD explosions or associated fires. It has been found that in the case of field vegetation burning, for example, that over 80% of the total suspended particulates have sizes of 1 micrometer or less in diameter. It has long been recognized that fine and smoke sized particles cause rapid clogging of the pore structures and, understandably, sampling at high volumetric rates accelerates dust loading of the sample filter and, where smoke or fine dusts are involved, filter clogging and sampling failure. This frequent shutdown and need for replacement of filters, is time consuming and expensive, and necessarily can interrupt an important monitoring and consequence management process.
Fine particulates exist naturally in many environments as well as a result of explosions. Among these are the very smoky conditions that develop in the vicinity of forest and range fires where fires and smoke are principal constituents of the ambient aerosol load. Elsewhere, energetic disturbances of soil that can accompany earth moving and construction activities can create high dust loads containing a large fraction of fines, and smoke particles are copiously generated by internal combustion engines, especially diesel engines. These conditions can quickly cause a prior art environmental continuous air monitor rapidly to experience clogged filters and a subsequent shut down of operation.
The present invention addresses the problem of rapid plugging of filters in high volume ambient continuous air monitors by selectively removing debris and fine particle components in the sampled ambient air before it passes through the sample collection filter, while at the same time, capturing the larger size particles that would be expected to contain the material of concern, such as the radioactive constituents in a radiological dispersal device (RDD) mixed in with fine dusts and smoke.
In order to achieve the objects and purposes of the present invention, and in accordance with its objectives, an inlet apparatus for removal of materials that preclude sustained, low maintenance operation comprises a pre-separator stage for eliminating suspended debris at the input to the system that might damage or degrade the performance of subsequent inlet stages, followed by a multiple nozzle virtual impactor stage having a 50% cut-point of about 2 micrometers aerodynamic diameter for removal without filtration of 90% or more of the fine particle and smoke constituents in the sampled air. Extracted environmental air passing through the pre-separator stage often contains re-suspended debris (leaves, flying insects, etc) that can be removed by inertial separation techniques to be deposited in traps, and then, upon exiting the throat of the first stage, passes over a deflector plate in a transition zone into a second stage consisting of a multiple nozzle dichotomous virtual impactor. Here in a third stage, the fine fraction of aerosol are removed and discarded without filtration, thereby achieving the objective of reducing the rate at which filter performance degrades. At the same time, nearly all of the larger size particles are transported to the filter and collected.